The preparation of cellulose ethers having uniform or different types of ether substituents is known [see, for example, "Ullmanns Ecyklopaedie der technischen Chemie" (Ullmann's Encyclopedia of Industrial Chemistry), Volume 9, keyword "cellulose ethers", Verlag Chemie-Weinheim, 4th edition 1975, page 192 et seq.], these being prepared, in general, either (a) by the principle of Williamson's ether synthesis by reacting cellulose with alkyl halides or aralkyl halides (with the stoichiometric consumption of a base) and/or (b) by reacting cellulose with activated reactants which are capable of directly reacting with hydroxyl groups in the presence of catalytic, i.e. sub-stoichiometric, quantities of a base: ##STR1## In these general equations:
Cell-O-H denotes, on the cellulose molecule, a hydroxyl group which is to be etherified,
Hal denotes chlorine or bromine,
R.sup.1 denotes a C.sub.1 to C.sub.15 alkyl radical, a C.sub.7 to C.sub.15 aralkyl radical, a carboxy(C.sub.1 -C.sub.3 alkyl) radical, a C.sub.1 to C.sub.3 sulfonoalkyl radical, a C.sub.1 to C.sub.3 phosphonoalkyl radical, a C.sub.1 to C.sub.6 hydroxyalkyl radical or an N,N-dialkylaminoalkyl radical in which each alkyl group has from 1 to 3 carbon atoms,
each of R.sup.2 and R.sup.3 denotes hydrogen (H) or a C.sub.1 to C.sub.13 alkyl radical, R.sup.2 being identical with R.sup.3 or different therefrom,
BOH denotes a base, such as alkali-metal hydroxide (e.g. NaOH) or alkaline-earth-metal hydroxide or a quaternary ammonium base.
For preparing mixed ethers of cellulose, various etherifying agents are allowed to act simultaneously or stepwise on cellulose. For this purpose, reactions according to only one of the indicated variants (a) or (b), but particularly reactions according to both variants, are carried out. The following are examples of reaction products which can be prepared by variant (a): methyl cellulose (MC), benzyl cellulose (BC), carboxymethyl cellulose (CMC), sulfonoethyl cellulose (SEC), phosphonomethyl cellulose (PMC), or N,N-diethylaminoethyl cellulose (DEAEC). The following are examples of reaction products which can be prepared by variant (b): hydroxyethyl cellulose (HEC), or hydroxypropyl cellulose (HPC). Mixed ethers of cellulose which can be prepared by any one or both of the indicated variant(s) include, for example, methyl hydroxyethyl cellulose (MHEC), ethyl hydroxyethyl cellulose (EHEC), hydroxyethyl hydroxypropyl cellulose (HEHPC), methyl carboxymethyl cellulose (MCMC), hydroxyethyl phosphonomethyl cellulose (HEPMC), or methyl hydroxyethyl hydroxypropyl cellulose (MHEHPC). Within the scope of the statements below, the term "cellulose ethers" includes both products having a unitary substituent, such as hydroxyethyl cellulose, and products having at least two different substituents, such as methyl carboxymethyl cellulose.
Most known processes for preparing cellulose ethers are carried out in two main steps:
1. The preparation of the "alkali cellulose". PA0 2. The etherification of the cellulose molecule. PA0 (a) alkalizing cellulose, PA0 (b) etherifying the alkalized cellulose in the presence of a base by using at least one etherifying agent which requires, for reaction with cellulose, a catalytic and sub-stoichiometric quantity of a base, PA0 (c) increasing the quantity of base, and PA0 (d) etherifying the cellulose ether so prepared by using at least one etherifying agent [which requires, for the reaction with cellulose, an at least stoichiometric quantity of a base or a catalytic and sub-stoichiometric quantity of a base which is increased over the quantity used in (a)], with at least one inert organic solvent being used as a dispersing auxiliary in at least one of the steps and water being present in all steps.
For preparing the "alkali cellulose", cellulose in a finely-divided (for example ground) form is mixed as homogeneously as possible in suitable technical equipment with water and alkali-metal hydroxide (in general NaOH, but other bases, such as quaternary ammonium bases, are also useful for this purpose). The alkali-metal hydroxide can be used in a solid form or in the form of an aqueous solution. For the etherification reaction itself and thus for the quality of the end product of the reaction, the uniformity and intensity of mixing is of decisive importance.
Alkalization is generally effected at as low a temperature as possible, for example, room temperature or below, in order to suppress degradation of the polymer (the so-called "ripening"); however, under certain circumstances, for example, the subsequent preparation of low-viscosity cellulose ethers, this degradation may be desirable. The etherifying agent is optionally added as early as the alkalization step, but in this case the temperature must generally be increased, in order to carry out the actual etherification reaction.
The actual etherification step is generally conducted by heating the alkali cellulose, produced in the first step, together with an etherifying agent (which has been added in the meantime) to temperatures between 30.degree. C. and 120.degree. C. It is also possible to remove, in advance, part of the water present in the first step. Vigorous mixing in the second step is also very important for the quality of the reaction product and for the cost-efficiency of the process, since, for example, it is desirable to have a good yield in the substitution reaction, while employing as small a quantity as possible of the etherifying agent(s).
Both continuous and discontinuous procedures are known for the two reaction steps. In the case of particular reactants, it is also possible to combine the two steps in such a way that pre-alkalization of the cellulose does not take place. Dispersing auxiliaries (suspending agents) are optionally employed in both steps, or at least in one of the two steps, in order to achieve better mixing of the heterogeneous reaction mixture. For this purpose, organic solvents which are either soluble in water or more or less insoluble in water are known from the state of the art, including, for example:
Ethylene glycol monoalkyl ether, ethylene glycol diethyl ether, dioxane, tetrahydrofuran, C.sub.1 to C.sub.6 alkanols (in particular isopropanol or tert.-butanol), (C.sub.1 to C.sub.4)alkoxy(C.sub.1 to C.sub.6)alkanols, toluene, heptane, mixtures of carbon tetrachloride and ethanol, acetone, methyl ethyl ketone; mixtures of benzene, toluene or xylene and ethanol; ethylene or propylene glycols, dioxane, mixtures of C.sub.6 and higher alkanes, aromatic compounds, aliphatic ketones, aliphatic ethers or C.sub.2 to C.sub.4 halogenated alkanes and C.sub.1 to C.sub.6 alkanols, dimethyl sulfoxide, dioxane or tetrahydrofuran; xylene or a mixture of tert.-butanol and acetone, mixtures of C.sub.5 to C.sub.10 alkanes or C.sub.6 to C.sub.12 aromatic compounds and C.sub.1 to C.sub.4 alkanols. Recently, an ethylene glycol diether, viz. dimethoxyethane, has been proposed for use as a new organic solvent in this field of application.
In German Offenlegungsschrift No. 3,147,434 of earlier priority date, which has not been previously published, a process for the preparation of cellulose ethers is described, which is carried out in the presence or water, bases, and at least one inert organic solvent comprising dimethoxyethane. According to another patent application (German Offenlegungsschrift No. 3,306,621 of earlier priority date, which has also not been previously published) a solvent mixture is employed in the preparation of cellulose ethers. This solvent mixture contains dimethoxyethane and, additionally, at least one further organic solvent, selected from the group consisting of alkanols, alkanediols, and alkoxyalkanols.
In practice, there is the problem that for the preparation of cellulose ethers according to the above-defined production variant (a), at least stoichiometric quantities of alkali-metal hydroxide must be used, relative to the desired degree of reaction of the alkyl halides or aralkyl halides. In the preparation of cellulose ethers according to production variant (b), on the other hand, only catalytic quantities of alkali-metal hydroxide are required.
As is known, too much alkali-metal hydroxide used in production variant (b) results in an impaired efficiency for the reaction with the etherifying agent. In this connection, efficiency is defined as the quotient of degree of substitution attained and total dosage of etherifying agent, multiplied by 100. When it is desired to prepare mixed ethers of cellulose and it is intended, for this purpose, to introduce, for example, two substituents, one of which is introduced according to production variant (a) and the other according to production variant (b), it is, for technical and economical reasons, necessary to conduct the process in such a way that the efficiency of reaction obtained with both etherifying agents is as high as possible. This is, however, contradicted by the actually required dosage of alkali-metal hydroxide, which must be practically stoichiometric for production variant (a). Examples of mixed ethers of this kind, in the preparation of which one substituent is introduced according to reaction variant (a) and the other according to reaction variant (b), are MHEC, MHPC, CMHEC or EHEC.
A similar absolute dependence also exists in the case of mixed ethers, in the preparation of which at least two different substituents are introduced, either exclusively according to variant (a) or exclusively according to variant (b). For this purpose, it may be necessary either to employ different quantities of alkali-metal hydroxide for the respective catalytically-influenced reaction or to use different conditions of reaction, because of widely varying reactivities of the etherifying agents. Examples of mixed cellulose ethers of this type are HEHPC, MCMC or HEHBC.
It has therefore been attempted to solve these problems by a number of processes which have been described in the past and in which, in the preparation of mixed cellulose ethers having, for example, two different substituents, etherification is carried out in two steps.
USSR-Pat. No. 397,519 describes a process for producing MHPC in two steps, by hydroxypropylation of a squeezed and disintegrated alkali cellulose (prepared with a 17 to 22% strength NaOH solution), at a ratio of propylene oxide to cellulose ranging from 0.9:1 to 1.5:1. NaOH in the form of a powder is then added in quantities of 0.5 to 0.7 part by weight per 1 part by weight of cellulose and, finally, methylation is carried out. It is stated that the reaction products comprise from 17 to 25% of hydroxypropyl groups and from 24 to 30% of methoxyl groups and that they are soluble in cold water and organic solvents; the process is discontinuously conducted. Disadvantages of the process are that (a) NaOH is used in the form of a powder, which, as is known, leads to a very irregular alkalization and thus also to products of mediocre quality, and (b) liquid dispersing auxiliaries are not used, which also results in products which are only non-uniformly etherified and show relatively high proportions of residues. Moreover, the process is apparently only suitable for the production of MHPC.
U.S. Pat. No. 4,096,325 discloses a process for preparing MHPC, in which alkali cellulose is first reacted with propylene oxide, in the presence of toluene, hexane, or DMF, at a ratio of propylene oxide to cellulose ranging from 1:1 to 8:1 and at a temperature of up to 110.degree. C. After a MS.sub.HP of about 0.5 to 7.0 has been attained, the organic solvent is mechanically removed (e.g. by filtering off or decanting). Upon adding fresh solvent, fresh NaOH, water, and methyl chloride, reaction is further conducted at 40.degree. to 75.degree. C., until a DS.sub.M of about 1 to 2.4 has been reached. According to example 2, the solvent may possibly be left in the product, but in that case the second etherification is already partly carried out in the first step. Disadvantages of this process are (a) the mechanical removal of the liquid components after the first step, which is hardly economical and is often detrimental to the product, (b) the substantial insolubility of the reaction products in pure water and also (c) the use of rather high-boiling organic solvents, some of which are, moreover, insoluble in water.
The two-step process of preparing water-soluble mixed cellulose ethers according to German Pat. No. 1,493,247 (equivalent of British Pat. No. 1,003,662) is carried out in such a way that (a) cellulose is alkalized with a 15 to 25% strength aqueous NaOH solution, (b) the alkali cellulose is squeezed off and is then reacted with gaseous alkyl halide or acrylonitrile, up to a DS from 0.05 to 0.5, (c) the amount of alkali is then reduced to less than 10% of the weight of the cellulose by washing with water and squeezing off, (d) the thus pre-treated cellulose is thereafter gradually reacted with gaseous alkylene oxide, until a MS of more than 1 is attained and (e) the unreacted alkylene oxide is finally removed; the remaining small amount of residual alkali in the product is neutralized in the gaseous phase. This process has the disadvantages that (a) liquid dispersion auxiliaries are not used, which leads to non-uniformly etherified products and (b) alkylation [as an example of the above-defined process variant (a)] is conducted in the first etherification step, so that the resulting intermediate product contains a comparatively high amount of residual alkali and must therefore be intermediately purified by washing before the second etherification step is carried out. Moreover, although it is maintained that etherification with gaseous etherifying agents yields better products, it is known to any person of ordinary skill in this field that a gas/solids interaction results in less uniformly etherified products than carrying out reaction with the aid of a dispersing auxiliary which is capable of dissolving the etherifying agent and thus renders possible a more intensive and more effective interaction between the components.
German Auslegeschrift No. 1,222,030 (equivalent of British Pat. No. 833,834) describes a method of preparing water-soluble and thermoplastic methyl-hydroxyalkyl celluloses, in which (a) cellulose is treated with an aqueous alkali-metal hydroxide solution of 30 to 60% strength until a weight ratio NaOH:cellulose in the range from 0.7 to 1.5 is attained and (b) the alkali cellulose is, successively or simultaneously, reacted with a hydroxyalkylating agent and methyl chloride, using, per part by weight of cellulose, from 0.25 to 0.8 part by weight of propylene oxide or molecularly equivalent amounts of ethylene oxide or from 0.15 to 0.8 part by weight of butylene oxide and from 1.1 to 2.0 parts by weight of methyl chloride. The reaction is first run at a temperature of up to 40.degree. C., which is then gradually increased to not more than 80.degree. C. This method has the disadvantages that (a) liquid dispersing auxiliaries are not employed, (b) a single alkalization step is carried out, so that the amount of alkali present before the hydroxyalkylation is too high, which leads to side reactions (hydrolysis of the alkylene oxide into alkylene glycols), and (c) the methyl chloride and propylene oxide used have a low percentage of activity of only 35% and 14%, respectively (according to example 1).